4.2: Meiosis and Sexual Life Cycles

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4.2 Meiosis and Sexual Life Cycles

• The process of creating sex cells
• i.e. Eggs (females)
• Sperm (males)

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Cell Reproduction

• Asexual (vegetative) reproduction
• A form of duplication using only mitosis.
• Example, a new plant grows out of the root or a
  shoot from an existing plant.
• Produces only genetically identical offspring
  since all divisions are by mitosis.
• Offspring called clones meaning that each is an
  exact copy of the original organism
• This method of reproduction is rapid and
  effective allowing the spread of an organism
• Since the offspring are identical, there is no
  mechanism for introducing diversity.

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Sexual reproduction

• Formation of new individual by a combination of
  two sex cells (gametes).
• Fertilization- combination of genetic information
  from two separate cells
• Gametes for fertilization usually come from
  separate parents
• Female- produces an egg
• Male produces sperm
• Both gametes are haploid, with a single set of
  chromosomes
• The new individual is called a zygote, with two
  sets of chromosomes.
• Meiosis is a process to convert a diploid cell to
  a haploid gamete, and cause a change in the
  genetic information to increase
  diversity/variation in the offspring.

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4.2.1 State that meiosis is a reduction division
of a diploid nucleus to form haploid nuclei.
Obj. 1

• The goal of meiosis is to make gametes.
• Gametes are sperm cells and egg cells

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Chromosomes
Chromosomes

• In humans, each somatic cell has 46 chromosomes.
• Made up of 23 pairs of homologous chromosomes?
  chromosomes that make up a pair that have the
  same length, centromere position and pattern of
  coded genes.
• The two chromosomes of each pair carry genes
  controlling the same inherited characters. (i.e.
  if a gene for eye colour is situated at a
  particular spot (locus) on a certain chromosome,
  then the homologue of that chromosome will also
  have a gene specifying eye colour at the
  equivalent locus.
• Except for the two sex chromosomes the
  chromosomes that determine an individuals sex
  (all other chromosomes are called autosomes.

How Do Scientists Read Chromosomes?
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4.2.2 Homologous chromosomes

• Homologous chromosomes two chromosomes that are
  the same size and show the same banding pattern.

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4.2.5 State that, in karyotyping, chromosomes are
arranged in pairs according to their size and
structure.(1).

• Make a Karyotype

Karyotype a visual representation of the
organization of the chromosomes in the cell of an
organism.
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Ploidy Number of sets of chromosomes in a cell

• Haploid (n)-- one set chromosomes. In humans
  n23
• Diploid (2n)-- two sets chromosomes. In humans
  2n2x2346
• Most plant and animal adults are diploid (2n)
• Eggs and sperm are haploid (n)
• Each gamete has a single set of the 22 autosomes
  plus a single sex chromosome, either X or Y.
  These are the only cells in the body not produced
  by mitosis.
• The formation of a gamete is considered a
  reduction division because the number of
  chromosomes is reduced by half.

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Chromosomes in a Diploid Cell

• Summary of chromosome characteristics
• Diploid set for humans 2n 46 (2 sets of 23)
• Autosomes homologous chromosomes, one from each
  parent (humans 22 sets of 2)
• Sex chromosomes (humans have 1 set of 2)
• Female-sex chromosomes are homologous (XX)
• Male-sex chromosomes are non-homologous (XY)

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What is Meiosis?
Figure Sexual Life cycle

• Meiosis involves two successive nuclear
  divisions that produce four haploid cells. The
  first division (meiosis I) is the reduction
  division the second division (meiosis II)
  separates the chromatids.
• Occurs in the ovaries or testes.
• Fertilization restores the diploid condition.

Gametes
n
2n
ZYGOTE
2n
Diploid multicellular organism
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Advantages of Meiosis

• Mitotic cell division produces new cells
  genetically identical to the parent cell.
• VS
• Meiosis increases genetic variation in the
  population. exchange of information can occur
  during meiosis I while mutations can also occur

Unique Features of Meiosis Comparison of Meiosis
and Mitosis
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4.2.3 Outline the process of Meiosis

• Interphase is an important stage preceding
  meiosis. Without this stage meiosis would not
  occur.
• During this stage, each individual chromatid
  replicates, similar to mitosis..
• At this stage, the chromosomes are long and
  stringy and are not visible.
• Remember All somatic cells are diploid in
  number (2n), therefore for each chromatid there
  also exists its homologue, which also replicates
  during interphase.

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Prophase I

• Prophase I is one of the most important stages of
  meiosis. The chromotid threads begin to twist
  and condense, creating chromosomal structures
  which are visible to the microscope.
• In a process called synapsis, homologous
  chromosomes, each made up of two sister
  chromatids, come together as pairs.
• After the homologous chromosomes pair, the
  structure is referred to as a tetrad (four
  chromatids). The point at which two non-sister
  chromatids intertwine is known as a chiasmata
  (sing. chiasma).
• Sometimes a process known as crossing over occurs
  at this point. This is where two non-sister
  chromatids exchange genetic material. This
  exchange does not become evident, however, until
  the two homologous pairs separate.(10.1.1
  Describe the behaviour of the chromosomes in the
  phases of meiosis. Obj. 2)

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10.1.2 Outline the process of the formation of
chiasmata during crossing over
Meanwhile, centrosomes move away from each other
and spindle fibres form between them. The nuclear
membrane disappears.
Animations
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Metaphase I

• At metaphase, each chromosome has reached its
  maximum density.
• The homologous pairs and their sister chromatids
  also prepare for separation.
• They interact with spindle fibers which form from
  either side of the nuclear envelope of the cell.
• There is a centriole at opposite ends of the
  cell, which is referred to as poles.
• During metaphase, the chromosomes are lined by
  the spindle fibers at what is known as the
  metaphase plate.

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Anaphase I

• Spindle fibres pull apart the tetrad, separating
  each homologous chromosome. Sister chromatids
  still remain attached to each other and move as a
  single unit toward the same pole.
• It is by random chance that a certain chromosome
  is pulled to a certain pole.

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Telophase I

• Telophase I varies from species to species.
• Sometimes Telophase I is skipped and meiosis
  starts its second division immediately.
• In general, however, two nuclear envelopes begin
  to surround the separate chromosomes and
  cytokinesis (splitting of the cytoplasm into two
  separate entities) will sometimes occur.
• Each pole now has a haploid chromosome set, but
  each chromosome still has two sister chromatids.
• Then a phase called interkinesis will follow,
  which essentially is a resting period from
  Telophase I to Prophase II.
• This differs from mitosis because DNA replication
  does not occur again.

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Prophase II

• During Prophase II, each dyad (1/2 a tetrad) is
  composed of a pair of sister chromatids and they
  are connected by a centromere.
• The centrioles (replicated during Telophase I)
  which produce the spindle fibers also start to
  move toward the poles of the cell.

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Metaphase II

• Metaphase II is similar to Metaphase I in that
  the dyads are lined up at a metaphase plate by
  the spindle fibers.

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Anaphase II

• The centromeres of sister chromatids finally
  separate, and the sister chromatids of each pair,
  now individual chromosomes, move toward opposite
  poles of the cell.
• Each sister chromatid ends up on one side of the
  cell.

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Telophase II

• At the end of Telophase II, the nuclear envelopes
  forms around each set of DNA at opposite poles of
  the cell and the cytoplasm divides once again
  (cytokinesis).
• As a result, four haploid daughter cells have
  formed from one diploid cell.
• The chromosomal content of a haploid cell in
  one-half the chromosomal content of a diploid
  cell (n as opposed to 2n)

Animations
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Cell Photograph of Telophase IIthe chromosomes
are ½ the number in a somatic cell
Stages of Meiosis
                

• End result of meiosis ? gametogenesis
  production of gametesSpermatogenesis process
  of male gamete production, one diploid cell gives
  rise to 4 sperm cellsOogenesis process of
  female gamete production, one diploid cell gives
  rise to 1 viable egg cell and three polar bodies,
  occurs in the ovaries once a month starting at
  puberty.

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Cdck- cell division activity
Meiosis
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• 10.1.3 Explain how meiosis results in an
  effectively infinite genetic variety in gametes
  through crossing over in prophase I and random
  orientation in metaphase I. Obj. 3
• 10.1.4 State Mendels law of independent
  assortment. Obj. 1

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Genetic Variation
Independent Assortment and Gamete
Diversity Random Orientation of Chromosomes
During Meiosis
Independent Assortment of Alleles

1. Independent Assortment of Chromosomes
   Arrangements of chromosomes are sorted out/moved
   to opposite poles by chance into gametes
   maternal and paternal (Metaphase I)
2. Crossing Over Combining DNA inherited from two
   parents into a single chromosome. (Prophase I)
3. Random Fertilization An egg cell has 1/8 million
   possible chromosomes combinations, is fertilized
   by a single sperm cell, which represents 1/8
   million different possibilities. (Potentially
   producing a zygote with any of 64 trillion
   diploid combinations).

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10.1.5 Explain the relationship between the law
of independent assortment and meiosis

• Independent assortment occurs during metaphase I
  of meiosis, when homologous chromosomes line up
  along the equatorial plane.
• As chromosomes sort randomly, they create
  opportunities for new recombinants during
  fertilization.

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4.2.4 Explain that non-disjunction can lead to
changes in chromosomes number

• Mutations can be inherited if an error occurs in
  the formation of one gamete and that gamete
  fertilizes another to form a zygote ? all the
  cells in the zygote will carry the mutation.
• Nondisjunction members of a pair of homologous
  chromosomes do not move apart properly during
  meiosis I, or in which sister chromatids fail to
  separate during meiosis II.
• One gamete receives 2 of the same type of
  chromosome and another gamete receives no copy
• Anueploidy abnormal chromosome number
• Trisomy if the chromosome is present in
  triplicate in the fertilized egg (2n1
  chromosomes) e.g. double in egg cell, normal in
  sperm cell e.g.Trisomy 21 Down Syndrome
• Monosomic if a chromosome is missing (2n-1)
• Polyploidy when organisms have more than 2
  complete chromosome sets (triploidy 3n,
  tetraploidy 4n) occurs if the cell fails to
  divide after replicating its chromosomes OR
  nondisjunction of all its chromosomes.

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Down Syndrome Karyotype
Using Karyotypes to Predict Genetic Disorders
3 of chromosome 21
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Other Errors
Syndrome Defect Problems
Petau Extra chromosome 13 Causes numerous physical and mental abnormalities, owing mostly to heart defects Lifespan is measured in days
Edward Trisomy 18 extra chrom 18 Usually naturally aborted or stillborn
Turner Females are missing X chromosome X The most common characteristics of Turner syndrome include short stature and lack of ovarian development. A number of other physical features, such as webbed neck, arms that turn out slightly at the elbow, and a low hairline in the back of the head are sometimes seen in Turner syndrome patients. Individuals with Turner syndrome are also prone to cardiovascular problems, kidney and thyroid problems, skeletal disorders such as scoliosis (curvature of the spine) or dislocated hips, and hearing and ear disturbances.
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More errors
Syndrome Defect Problem
Triplo-X Females have an extra X chromosome XXX The symptoms of this syndrome vary widely.  The one consistent feature is tall stature.  Some females exhibit no or very few symptoms, while others have more severe features of developmental delay and/or behavioral abnormalities.
Klinefelter Males have an extra X chromosome XXY XXY males usually have difficulty with expressive language the ability to put thoughts, ideas, and emotions into words. In contrast, their faculty for receptive language-understanding what is said-is close to normal.
Jacobs Males have an extra Y XYY XYY males are fertile, they have testes of normal size, and they have a normal sexual libido and potency. In spite of a somewhat decreased sperm quality with many so-called immature sperm cells, the fertility seems to be normal. In the same way as for triple-X women, who as a rule do not get children with an extra X chromosome, males with XYY most probably only very rarely get sons with two Y chromosomes.
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4.2.6 State how karyotyping is performed

• Karyotyping is performed using cells collected by
  chorionic vilus sampling or amniocentesis, for
  pre-natal diagnosis of chromosome abnormalities.

http//www.massasoit-bio.net/courses/136/136_cours
eassets/cummings_animations/karyotype.html
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• 4.2.7 Analyse a human karyotype to determine
  gender and whether non-disjunction has occurred.

http//learn.genetics.utah.edu/content/begin/trait
s/predictdisorder/